Recently, along with the advancement of basic technology, the development of optical disks originating from compact disks (CDs) has lead to a significant increase in the memory size per unit area of the disk. This typical basic technology includes shortening the frequency of the light source, increasing the numerical aperture of the objective lens, and developing a more efficient recording mode.
Such an increase in the recording density of an optical disk has led to innovative new products. One such product is a CD-size optical disk having a large recording capacity. Such an optical disk is anticipated to function as an image recording device or computer memory device for recording several hours of high-definition images without deteriorating the quality of the image. Another optical disk developed through the increase in the recording density of optical disks is a small diameter optical disk having a sufficient memory capacity.
In particular, use of this small diameter optical disk in the field of portable apparatuses is expected. For example, the small diameter optical disk may be installed in a camcoder, a notebook computer, a personal digital assistance (PDA), a digital camera, or a portable game device. The small diameter optical disk allows the size of the portable apparatuses to be reduced and to run an application that requires a large data capacity, which have not been possible using known technology.
There are several technical difficulties in increasing the recording density of a small diameter optical disk used for portable apparatuses. One difficulty is to develop a small optical head corresponding to the small diameter optical disk.
When the numerical aperture of the objective lens is increased by employing a basic technology for realizing a high-capacity optical disk by increasing the recording density, in exchange for increasing the capacity, the effect of dust contaminating the optical disk becomes large. Therefore, dust control of the optical disk becomes essential, and it becomes necessary to store the optical disk inside a cartridge. Increasing the numerical aperture of the objective lens causes the working distance between the objective lens and the optical disk to become smaller. Due to these two factors, in an optical disk system, the optical head that holds the objective lens and that can be moved to a predetermined position must be of a size small enough to be stored inside the opening for the cartridge (shutter opening window).
On the basis of the description above, when using a small diameter optical disk, the opening for the cartridge becomes small since the diameter of the optical disk is small. As a result, it becomes necessary to develop an optical head having a significantly small size.
A known optical head for an optical pick-up will be described by referring to the drawings.
FIG. 8 is a perspective view of an example of a known optical head including a two-axis actuator of an open magnetic path.
The optical head illustrated in FIG. 8 includes a coil bobbin 14, an objective lens 15, a focusing coil 16, a pair of tracking coils 17a and 17b, four flat springs 18a to 18d, a support 19, magnets 20a and 20b, and yokes 21a and 21b. 
The objective lens 15 is supported at the center of the coil bobbin 14 by aligning the optical axis with the focusing direction (Z axis). The focusing coil 16 is disposed on the periphery of the coil bobbin 14 so that the focusing coil 16 is wound around the Z axis extending in the focusing direction. Furthermore, the tracking coils 17a and 17b are rectangularly wound around the X axis at the ends of the coil bobbin 14 in the tracking direction (X axis), which is the direction orthogonal to the optical axis of the objective lens 15.
The coil bobbin 14 including the objective lens 15 is supported by the support 19 with four flat springs 18a to 18d so that the objective lens 15 can oscillate in the focusing direction (Z axis) and the tracking direction (X axis).
The coil bobbin 14 is interposed between the yokes 21a and 21b, wherein the yokes 21a and 21b are disposed so that they are vertical to the Y axis and oppose each other in the Y axis direction orthogonal to the focusing direction (Z axis direction) and the tracking direction (X axis direction). A pair of magnets 20a and 20b of the same pole (e.g., north pole), are disposed on the yokes 21a and 21b so that they oppose each other. The coil bobbin 14 including the focusing coil 16 and tracking coils 17a and 17b is disposed within the magnetic field generated by the magnets. 20a and 20b. 
In such an optical head, by applying an electrical current to the focusing coil 16 that is orthogonal to the magnetic field component in the Y axis direction of the magnets 20a and 20b, a driving force in the focusing direction (Z axis direction) is applied to the coil bobbin 14 including the objective lens 15. By applying an electrical current to the tracking coils 17a and 17b orthogonal to the magnetic field component in the Y axis direction of the magnets 20a and 20b, a driving force in the tracking direction (X axis direction) is applied to the coil bobbin 14 including the objective lens 15.
FIG. 9 is a perspective view of another example of a known optical head including a two-axis actuator having a closed magnetic path.
The optical head illustrated in FIG. 9 includes a chassis 22, an objective lens 23, a focusing coil 24, a pair of tracking coils 25a and 25b, four flat springs 26a to 26d, a support 27, a magnet 28, a yoke 29, and a back yoke 30.
The coil bobbin 22 extends in the Y axis direction, which is perpendicular to the focusing direction (Z axis). At the tip of the coil bobbin 22, the objective lens 23 is supported. The focusing coil 24 is disposed inside an opening 221 formed at the rear edge of the coil bobbin 22 and is rectangularly wound around the Z axis in the focusing direction. The tracking coils 25a and 25b are rectangularly wound around the Y axis and are disposed in parallel in the tracking direction (X axis) so that they are in contact with the inner circumference of the focusing coil 24 on the side closer to the objective lens 23.
The coil bobbin 22 including the objective lens 23 is fixed to the support 27 by the four flat springs 26a to 26d so that the coil bobbin 22 can oscillate in the focusing direction (Z axis) and the tracking direction (X axis).
The yoke 29 is disposed orthogonally to the Y axis at a position close the support 27 on the inner side of the focusing coil 24 in the Y axis direction. The magnet 28 is disposed on the yoke 29. The back yoke 30 is disposed orthogonally to the Y axis in the opening 221, close to the objective lens 23 positioned on the outer side of the focusing coil 24 in the Y axis direction.
In the optical head illustrated in FIG. 9, similar to the optical head illustrated in FIG. 8, by applying an electrical current to the focusing coil 24 orthogonal to the magnetic field component in the Y axis direction of the magnet 28, a driving force in the focusing direction (Z axis direction) is applied to the coil bobbin 22 including the objective lens 23. In this case, because the back yoke 30 is disposed, the magnetic flux density increases. Moreover, the magnetic flux passes through the coil side contributing to the driving of the focusing coil 24 and forms a magnetic field distribution through the back yoke 30. In this way, the driving force generated in the opposite direction is reduced by magnetic flux lines passing through other coil sides.
The principle of driving the coil bobbin 22 including the objective lens 23 in the tracking direction will be described below.
One of the sides of the tracking coils 25a and 25b according to this embodiment is disposed orthogonally to the magnetic flux lines in the Y axis direction of the magnet 28 to generate a forward driving force in the tracking direction. Therefore, in this case, to prevent the generation of the backward driving force from second sides parallel to first sides of the tracking coils 25a and 25b, the tracking coils 25a and 25b are disposed plane-symmetrically with respect to the plane including the Y and Z axes and are center-displaced from the Y axis orthogonal to the optical axis of the objective lens for one back yoke such that magnetic flux lines are orthogonal to the fist sides of the tracking coils 25a and 25b and do not propagate to the second sides of the tracking coils 25a and 25b. 
For such an optical head having a closed magnetic path, the magnetic circuit is disposed only on one side. Thus, the size of the optical head can be reduced in the Y axis direction.
FIG. 10 is a perspective view of another example of a known optical head including an axial-sliding-type two-axis actuator.
The optical head illustrated in FIG. 10 includes a coil bobbin 31, an objective lens 32, a focusing coil 33, a pair of tracking coils 34a and 34b, tracking magnets 35a and 35c, focusing magnets 35b and 35d, tracking yokes 36a and 36c, focusing yokes 36b and 36d, back yokes 37a and 37b, a shaft 38, and a counter balance 39.
The center of the circular coil bobbin 31 is attached to the shaft 38 protruding from the fixed portion in the focusing direction (Z axis direction) so that the coil bobbin 31 is rotatable around the shaft 38 and slidable on the shaft 38 in the focusing direction (Z axis direction). The coil bobbin 31 has an objective lens 32 decentered in the Y direction. Furthermore, the counter balance 39 is disposed on the opposite side to the objective lens 32.
The focusing coil 33 is wound around the external periphery of the coil bobbin 31. The tracking coils 34a and 34b are disposed on the ends of the coil bobbin 31 in the Y axis direction.
The tracking yokes 36a and 36c are disposed on the ends of the coil bobbin 31 in the Y axis direction so that the tracking yokes 36a and 36c oppose each other. On the inside of the tracking yokes 36a and 36c, the tracking magnets 35a and 35c are attached, respectively. The focusing yokes 36b and 36d are disposed on the ends of the coil bobbin 31 in the X axis direction. On the inside of the focusing yokes 36b and 36d, the focusing magnets 35b and 35d are attached, respectively.
The back yokes 37a and 37b are disposed on the inside of the coil bobbin 31 so that they oppose the focusing magnets 35b and 35d, respectively.
For such an axial sliding type optical head, illustrated in FIG. 10, by applying an electrical current to the focusing coil 33, the coil bobbin 31 moves in the Z axis direction relative to the shaft 38. In this way, the objective lens 32 moves in the focusing direction. By applying an electrical force to the tracking coils 34a and 34b, the coil bobbin 31 rotates around the shaft 38. In this way, the objective lens 32 moves in the tracking direction.
For a known optical disk system in which the size of the optical disk such as a CD or a DVD is 120 mm in diameter, the size of the optical head does not need to be reduced. Moreover, for a small diameter optical disk system such as an MD, the optical head does not necessarily have to be disposed inside an opening of an optical disk cartridge because the numerical aperture of the objective lens is not large and, therefore, the distance between the objective lens and the optical disk is large. For these reasons, the size of the optical head does not need to be reduced.
Although the optical head does not need to be stored in the opening of the optical disk cartridge, when a small optical head is to be used for a portable apparatus, the dynamic performance of the two-axis actuator of the optical head does not correspond to the high density recording disk format. To increase the recording density of the optical disk, the margins for defocusing and detracking are reduced and the sensitivity and frequency of the actuator are increased along with the increase in the transfer rate.
As described above, although a reduction in size and dynamic performance are required for the two-axis actuator, or, in other words, the optical head, that is used for an optical disk with a small diameter and a high recording density, known optical heads cannot meet these requirements.
In other words, the known optical head illustrated in FIG. 8 is not suitable for reducing the size because the magnets 20a and 20a of the same pole must be disposed along the Y axis, which is orthogonal to the optical axis of the objective lens 15, so that the magnets 20a and 20b oppose each other.
For the known optical head illustrated in FIG. 9, the size may be reduced by disposing the magnetic circuit on one side. By reducing the size, however, a secondary resonance of the movable parts including the objective lens and the coil bobbin decreases and the dynamic performance becomes unbalanced because of the difference in the positions of the center of gravity, the driving point, and the support point, making it difficult to improve the performance of the optical head.
For the known optical head illustrated in FIG. 10, the size may be reduced and the performance may be improved. In such a case, however, the linearity in the fine driving is not maintained because of the friction between the shaft and the shaft hole on the bobbin. Therefore, there is a problem in that the optical head is not suitable for an optical disk system with small defocusing and detracking margins.
An object of the present invention is to solve the above-mentioned problems and provide an optical head whose size can be easily reduced to a size that can be stored in an opening of a cartridge for an optical disk and whose dynamic performance can be easily improved along with an increase in recording density and transmitting rate.